feat(format): add color format 'xyz'

[Tieske]

replaces #255 the color part

[Tieske]

I added a second commit that will allow a property to support multiple color formats, eg, RGB + XYZ. This will enable homie-library authors to write the color conversion once, and the device author (using the library) will not have to write his own.

For example;

• device supports RGB
• homie lib supports RGB + HSV -> publishes device as supporting RGB+HSV
• controller sends HSV value
• lib convert received HSV to RGB
• device consumes RGB

Obviously this is just a suggestion, it can be removed again.